[grid] Fix running the benchmark with a non supported system

In certain cases, e.g. when running on a non supported system, attributes like
`current_partition.processor.num_cpus` and
`current_partition.processor.num_cpus_per_core` have value `None`, which breaks
some arithmetic operations, so we use `VAR or 1` to give it a "default" value of
1.  Note that all these attributes are used to the set the default values of
other attributes, but they can all be overridden on the command line.

benchmarks/apps/grid/grid.py

@@ -66,10 +66,10 @@ def setup_num_tasks(self):
         self.num_tasks = reduce(mul, list(map(int, self.mpi.split("."))), 1)
         self.set_var_default(
             'num_cpus_per_task',
-            self.current_partition.processor.num_cpus //
-            min(1, self.current_partition.processor.num_cpus_per_core))
+            (self.current_partition.processor.num_cpus or 1) //
+            min(1, self.current_partition.processor.num_cpus_per_core or 1))
         self.set_var_default('num_tasks_per_node',
-                             self.current_partition.processor.num_cpus //
+                             (self.current_partition.processor.num_cpus or 1) //
                              self.num_cpus_per_task)
         self.executable_opts = [f'--mpi {self.mpi}', f'--shm {self.shm}', '--shm-hugetlb']
         self.env_vars['OMP_NUM_THREADS'] = f'{self.num_cpus_per_task}'